This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Over the past 30 years the prevalence of overweight children (6-19 y) in the USA (BMI>95th percentile) has increased 3-fold to reach 13-14% in 1999. Concomitant with the increase in childhood overweight, the incidence of type 2 diabetes in children (0-19 y) has increased 4-fold. Although the etiology of obesity related impairment of glucose metabolism is multifactorial, dietary macronutrient intake and physical activity are potentially important factors. The aims of this proposal are to determine in healthy adolescents 1) the impact of obesity on energy expenditure and substrate oxidation, glucose production from gluconeogenesis and glycogenolysis, insulin secretion and sensitivity and lipolysis;2) whether a 12 weeks moderate exercise program will improve glucose metabolism in obese adolescents, and if so, 3) whether the exercise effect is correlated with a decrease in intramyocellular fat content and/or increased lipid oxidation, and 4) whether dietary macronutrient composition of the diet impacts metabolic effects of exercise. To address these questions, 24 over weight (body fat >30%) and 24 non-obese (body fat <26%) adolescents will be studied on three occasions (just prior to the start of a 12 weeks exercise program, 6 weeks into the program and during the last week of exercise);in 24 subjects,12 obese and 12 non-obese, each of the three study occasions will be preceded by 7-d of a high carbohydrate diet, and in the other 24 of a 7-d high fat diet. State of the art stable isotope-GCMS techniques, MRI and 24 h room calorimetry will be used to obtain the data needed to address the aims stated above. The information provided by these studies will improve our understanding of the metabolic effects of obesity in adolescents and will lead to identifying factors, which may increase the risk of type 2 diabetes or prevent or delay the development of type 2 diabetes in adolescents.